When a voltage is applied to an organic electroluminescence device (hereinafter referred to as an organic EL device), holes are injected from an anode into an emitting layer and electrons are injected from a cathode into the emitting layer. The injected holes and electrons are recombined in the emitting layer to form excitons. Here, according to the electron spin statistics theory, singlet excitons and triplet excitons are generated at a ratio of 25%:75%. In the classification according to the emission theory, in a fluorescent organic EL device which uses emission caused by singlet excitons, the limit value of an internal quantum efficiency is believed to be 25%. On the other hand, in a phosphorescent EL device which uses emission caused by triplet excitons, it has been known that the internal quantum efficiency can be improved up to 100% when intersystem crossing efficiently occurs from the singlet excitons.
In a typical organic EL device, the most suitable device design has been made depending on fluorescent and phosphorescent emission mechanism. Particularly, when a fluorescent device technique is simply used for designing the phosphorescent organic EL device, it has been known that a highly efficient phosphorescent organic EL device cannot be obtained because of a luminescence property of the phosphorescent organic EL device. The reasons are generally considered as follows.
First of all, since the phosphorescent emission is generated using triplet excitons, an energy gap of a compound for the emitting layer must be large. This is because a value of an energy gap (hereinafter, occasionally referred to as singlet energy) of a compound is typically larger than a value of triplet energy (herein, referred to as an energy gap between energy in the lowest triplet state and energy in the ground state) of the compound.
Accordingly, in order to efficiently trap triplet energy of a phosphorescent dopant material in the device, a host material having larger triplet energy than that of the phosphorescent dopant material needs to be used in the emitting layer. Moreover, an electron transporting layer and a hole transporting layer need to be provided adjacently to the emitting layer. A compound used as the electron transporting layer and the hole transporting layer needs to have a larger triplet energy than that of the phosphorescent dopant material.
When the phosphorescent organic EL device is thus produced according to the designing idea of the typical organic EL device, a compound having a larger energy gap than that of a compound used as a fluorescent organic EL device is used, thereby increasing drive voltage of the overall organic EL device.
Although a hydrocarbon compound exhibiting a high oxidation resistance and a high reduction resistance is useful for the fluorescent device, the hydrocarbon compound has a broad it electron cloud to render the energy gap small. For this reason, such a hydrocarbon compound is unlikely to be selected for the phosphorescent organic EL device, so that an organic compound including a hetero atom (e.g., oxygen and nitrogen) is selected. Consequently, a lifetime of the phosphorescent organic EL device is shorter than that of the fluorescent organic EL device.
Moreover, device performance of the phosphorescent organic EL device is greatly affected by an exciton relaxation rate of triplet excitons much longer than that of singlet excitons in the phosphorescent dopant material. In other words, with respect to emission from the singlet excitons, since a relaxation rate leading to emission is so fast that the singlet excitons are unlikely to diffuse to the neighboring layers of the emitting layer (e.g., the hole transporting layer and the electron transporting layer), efficient emission is expected. On the other hand, since emission from the triplet excitons is spin-forbidden and has slow relaxation rate, the triplet excitons are likely to diffuse to the neighboring layers, so that the triplet excitons are thermally energy-deactivated unless the phosphorescent dopant material is a specific phosphorescent compound. In short, in the phosphorescent organic EL device, control of the recombination region of the electrons and the holes is more important as compared with the control in the fluorescent organic EL device.
For the above reasons, enhancement of the performance of the phosphorescent organic EL device requires material selection and device design different from those of the fluorescent organic EL device.
As an application of the luminous material for realizing the phosphorescent organic EL device having a high luminous efficiency, Document 1 (International Publication No. 2008/056746) discloses an organic EL device having a phosphorescent emitting layer including a specific compound with an indocarbazole skeleton as a host material and a phosphorescent compound (Ir(ppy)3) as a dopant material. Document 1 describes that such an arrangement of the emitting layer allows efficient emission from the dopant material to improve the luminous efficiency of the organic EL device as compared with an arrangement in which the host material is Alq3.
When the specific compound with an indocarbazole skeleton is used as the host material in the phosphorescent emitting layer as described in Document 1, the luminous efficiency of the organic EL device is improved only in comparison with the arrangement in which the host material is Alq3. However, the luminous efficiency is not sufficient for practical use. Accordingly, further enhancement of the luminous efficiency has been desired. Moreover, Document 1 only examines an organic EL device in which Alq3 having an insufficiently large electron mobility is used as an electron transporting layer. Accordingly, an electron transporting layer exhibiting a large electron injecting performance to the emitting layer has been desired in view of application of an emitting layer system exhibiting a high luminous efficiency.